U.S. Pat. No. 4,111,051 describes method and apparatus for sample feeding systems utilized in flameless atomic absorption spectroscopy, in which a sample is withdrawn from a sample vessel by a dosing tube and then the dosing tube is moved to an atomizing device, e.g. a graphite tube atomizer and the sample is dispensed therein. The other end of the dosing tube communicates with a sample pump and with a rising fluid pump for delivery in one direction only. A rinsing process is employed between the individual dosing processes if successively different samples are dosed, wherein the free end or tip of the dosing tube is dipped into a rinsing vessel. Rinsing liquid is pumped by the rinsing liquid pump from a rinsing liquid container through the dosing tube into the rinsing vessel, which is formed as an overflow vessel, whereby the tip of the dosing tube is rinsed on its inside as well as on its outside. The dosing tube is filled with rinsing liquid and a small air volume is taken in by the sample pump or a separate air pump after the dosing tube has been lifted out of the rinsing vessel. Then the tip of the dosing tube is guided to a sample vessel and a well-defined sample volume is taken in by means of the sample pump. This sample volume is supplied to the atomizing device by a return stroke of the sample pump. A return stroke of the air pump ejects at least a part of the air volume taken in, whereby residue of the sample liquid is blown off the wall of the dosing tube.
The rinsing liquid pump comprises a pump cylinder having an inlet and an outlet. A pump piston is arranged in the pump cylinder, which is movable by a servomotor between a first end position in the discharge direction and a second opposite end position in the intake direction. One check valve is arranged on the inlet side and a second check valve is arranged on the outlet side of the pump cylinder. Rinsing liquid is taken in through the check valve in the pump cylinder on the inlet side with the intake stroke of the pump piston. This rinsing liquid is forced into the dosing tube through the check valve on the outlet side during the subsequent discharge stroke.
Very small sample volumes are dosed with such devices which, for example, is very desirable in flameless atomic absorption spectroscopy. It is very important that these sample volumes be observed very exactly, because atomic absorption spectroscopy is a quantitative measuring method, and depends on the quantity of atoms of a looked for element in a cloud of atoms generated in the graphite tube and as a result, an error in the dosed sample volume directly and substantially affects the end results.
Similar problems also appear in the high pressure liquid chromatography.
It has been found that the test results are sometimes inconsistent when employing the above described testing methods. It is one object of the present invention to avoid these measuring errors.
One of the aspects of the invention is based on the discovery that the described measuring errors are due to leakages of the check valves in the rinsing liquid pump. Such check valve leakage can be caused in the outlet of the rinsing liquid pump by particles in the rinsing liquid. Also, the surface tension of the rinsing liquid can affect the valve. These leakages become more apparent when the dosed sample volume is small.